Structure analyses will be directed at understanding the molecular basis of specific interactions between proteins and lipids. Continued work is planned on protein-ligand systems which are currently under study by high resolution single crystal X-ray diffraction; (1) phospholipase A2 from C. atrox (PLPA2) which interacts with phospholipids condensed in a membrane-like state; (2) delta 5--3-ketosteroid isomerase (KSI) which binds specific steroids and catalyzes the transfer of a double bond. Studies will be extented to include the stereochemical analysis of interaction with substrate analogues, competitive inhibitors (both PLPA2 and KSI) and metallic cofactors (PLPA2). Efforts are planned to crystallize analogues of PLPA2 (e.g., bee venom PLPA2 and the neurotoxin, Crotoxin) and KSI (e.g., the steroid transport protein of P. testosteroni and human plasma) and thereby enlarge the data base from which stereochemical conclusions can be drawn.